1. Field of the Invention
The present invention relates to a distance measuring apparatus. More particularly, this invention is concerned with a distance measuring apparatus designed for cameras to detect a distance from a subject for photography.
2. Description of the Related Art
As already known, the distance measuring apparatus for cameras to measure a distance from a subject by processing a signal originating from a subject image formed on an image sensor or other photoelectric transfer means is broadly divided into a passive type that uses brightness distribution information of a subject to calculate the distance from the subject and an active type that an infrared ray or ultrasonic beam is projected to a subject, and then the reflected signal is detected to measure the distance from the subject. Whichever type is adopted, a signal to be handled is so weak that distance measurement is always accompanied by a problem of deteriorated accuracy.
An active type triangular distance measuring apparatus will be described as an example. An arithmetic logic output of distance measurement is provided as a single characteristic line L1, meaning that the arithmetic logic outputs are proportional to the reciprocals of distances from a subject as shown in FIG. 7. However, the signal reflected from a subject is provided as a very weak photocurrent, causing the quantity of light in the reflected light to lessen as small as several tens of picoamperes at a far distance. That is to say, the arithmetic logic outputs of the distance measurement are unreliable because of shot noise characteristic of a photocurrent detector or a photocurrent, or noise characteristic of a sensor for detecting the reflected signal.
For example, the line L1 shown in FIG. 7 resides in a band-like shaded zone of unreliability No which is sandwiched by curves L2 and L3 shown in FIG. 8. Therefore, the arithmetic logic outputs are represented as the values existent in a certain probability space. Specifically, the relationship between the outputs and the reciprocals of distances is not recognized as 1:1. As a result, the graph of arithmetic logic outputs vs. distance reciprocals does not become a single line.
The aforesaid unreliability may increase with the influence of illumination equipment including fluorescent lamps and neon signs, or other AC background light.
As an effective means for reducing the unreliability No and improving accuracy in distance measurement, for example, U.S. Pat. No. 5,005,970, U.K. Patent No. 2,212,688, and U.S. Pat. No. 5,001,508 have disclosed an integrating means for measuring a distance several times (n times), integrating distance measurement data, then averaging the data (this is a known means used for measuring instruments for detecting weak signals). According to this means, unreliability can be lessened to 1/n.sup.1/2 by performing distance measurement n times.
The integrating means is a very efficient signal processor, which is applicable to all active type distance measuring apparatuses. However, when the integrating means applies to a distance measuring apparatus for cameras, it poses a critical drawback. That is to say, it takes much time to measure conventional n-fold distances. This is a fatal drawback for cameras which must ensure timely shutter chances.
Sufficient time is required to measure a distance multiple times. This results in missing shutter chances even when priority has been given to shutter timings but not to correct focus control.